If you add an Frame of the type=image you can choose the texture it should have with the <Texture val="YourTexture"/>. Furthermore if you want it to become a border, much like the dialog items (image) created with triggers, you can use <TextureType val="Border" layer="0"/>.
Here an example:

Re the original post, yes, people are using this post 1.2 and your tutorial is appreciated :) At first I started trying to remove frames that I didn't want, and was baffled by the crashes. Your tutorial and demo files helped get me unstuck, and I'm sure others too.

As for useful wiki information, I think something that probably confuses most people is simply which element corresponds to what. A graphical breakdown of which elements in the UI files correspond to which element of which dialog would probably help people identify the elements they're looking to move/remove. Would be a bit of a hassle to fully document that, but in addition to what you already have, it seems the next most useful thing you could document.

But my problem is, I want to have it just as a mask behind the minimap etc without beeing attached to a specific template.
The second problem is, where the heck do I get the GameUIData.xml?
And I'm also not quiet sure about the trigge, guess it's something like create a dialog with x/y blabla then hiding it's background and then show something ^^

So it would be just awesome if someone can explain me how I get this working step by step so that a noob like me understand it :D
Thanks so far

Thanks so far, guess I will do it like you said ...
But whats the path of the picture then? I imported it but thats all, so it's in no specific folder :o
Shall I import it in the same folder where the sc2layout files are lying and whats with this GameUIData.xml? Where is it and what do I have to do with it?

Thanks :)

€dit:
OK, I've downloaded a mpqeditor and found the GameUIData.xml, added '<CustomLayoutFileArray value="ui_02.dds"/> after the Dflt entry and nothing changed as far as I can see it :o
Correct so far? What's next :D

ps. I just wrote this out from memory. it can contain values that are incorrect.
it should generate an image which positions itself accordingly to the minimap.
This is placed inside a SC2Layout file which you then import into your map

After you've imported it and saved the map you go to the data editor to the Game UI Data type. then at the Dflt entry, at the fields scroll down until you see Custom Layout File. there add the just imported SC2Layout file to the list. and remove your ui_02.dds

I'll see if I have some time to generate the correct code but I probably won't have it. :(

Oh, now I just realized someone answered :D
Thanks m8, but got it working, I just replaced the gas icon (I don't need it) and changed its position as well as its size. I tried doing a simple plane in 3ds max with my texture on it and replacing the original ui what didn't work, so I went this way...
So as you can see in this video it worked fine but now after the patch everything is destroyed, thx blizz -.-